Concrete is generally produced from the following components:                aggregates, comprising sand and gravel, the particle sizes of which can vary according to the desired grade of the concrete;        cement;        one or more optional admixtures; and        water.        
To produce the concrete, it is known to use a production plant installed on the work site where the concrete is intended to be used.
This plant comprises:                sand and gravel storage containers associated with a shelf for withdrawing the aggregates from each storage container;        at least one silo for storing the cement;        optionally, another silo intended to receive fly ash or filler to be added to the mixture; and        optionally, another storage container for a liquid admixture.        
At the moment of use, the concrete is produced by mixing the components with water.
It is imperative to measure and set, each day, the degree of moisture of the sand so as to deduce the amount of water incorporated into the concrete in order to keep the water/cement ratio below a specified value, for example 0.60 in the case of a C25/30 cement type.
This limit is imposed by regulations, in order to allow the plasticity of the concrete to be controlled.
Since the mixture is produced by the plant on site, it is necessary to control and set many parameters relating to this mixture, and also the amount of water and the power absorbed by the mixer by means of a controller, which also delivers time data relating to the mixing time.
Such a plant must be checked by a qualified person, as dictated by the regulations.
This type of plant also takes up a very large amount of space owing to the presence of the storage tanks. Moreover, the storage tanks must be protected from moisture, thereby complicating their maintenance.
According to another known method, the concrete is prepared in a plant at some distance from the work site and brought to the work site by trucks called “truck mixers”.
This second solution imposes considerable logistic constraints since the delay before using the concrete contained in the truck mixer is limited and the organization of the work on the work site is determined by the arrival of the trucks.